1. Field of the Invention
The present invention relates to an operating apparatus (in other words, a lightening apparatus) for a discharge lamp, and in particular to an operating apparatus for reducing a starting voltage of the discharge lamp by intentionally depositing a material enclosed in the discharge lamp on one of two electrodes in the discharge lamp.
2. Description of the Invention
HID lamps have been more and more widely used recently for their features of, for example, high luminance, high efficiency and long life. In particular, metal halide lamps have been used as a light source for indoor lighting, light sources for video equipment, and headlights of automobiles, in view of their satisfactory color rendering characteristics.
A discharge lamp requires a ballast circuit for generating a starting pulse for starting discharge and supplying power for maintaining the operating state.
Generally, a voltage required for starting operation (i.e., discharge) of the lamp (hereinafter, referred to as the "starting voltage") is significantly higher than the voltage required when the lamp is operating under the rated conditions, and typically needs to be as high as several kilovolts to several tens of kilo-volts. Accordingly, a ballast circuit includes a starting pulse generator for generating such a high level of starting pulse when the operation is to be started. However, the starting pulse generator requires sufficient insulation since it generates such a high level of voltage. Accordingly, the starting pulse generator occupies a large area, despite operating only when discharge is started. The starting voltage needs to be lowered in order to reduce the size of the starting pulse generator.
A method for lowering such a high starting voltage is disclosed in, for example, Japanese Laid-Open Publication No. 51-66174. According to the method disclosed in this publication, a radioactive material is enclosed in the discharge lamp, and ionization of the gas in the discharge lamp is facilitated by the radioactive rays generated by the radioactive material, thereby reducing the starting voltage.
Japanese Patent Application No. 6-265318 (corresponding to Japanese Laid-Open Publication No. 8-130096) and Japanese Laid-Open Publication No. 7-146515 each disclose a method for reducing the starting voltage by preventing the material enclosed in the discharge lamp (hereinafter, referred to as an "enclosed material") from depositing on the electrodes.
In general, the starting voltage is lower when no enclosed material is deposited on the electrodes than when some of the enclosed material is deposited on the electrodes. This is considered to be resulted because the work function changes at the tip of the electrode by the enclosed material depositing thereon.
A metal material mainly containing tungsten has a smaller heat capacity than that of quartz glass forming the arc tube. Thus, when the discharge lamp is turned off, the temperature of the electrodes is lowered more rapidly than that of the arc tube. The enclosed material in the discharge lamp, which is in an evaporated state while the lamp is operated, can condense and stay stably on a surface where the temperature is sufficiently lowered. Since the temperature of the electrodes is first lowered, the enclosed material deposits on the electrodes.
Accordingly, the next time when the discharge lamp is operated, discharge is started in the state where the enclosed material deposits on the electrodes. As a result, the starting voltage is raised. The enclosed material depositing on the electrodes can be easily confirmed by visual inspection.
Japanese Patent Application No. 6-265318 (corresponding to Japanese Laid-Open Publication No. 8-130096) and Japanese Laid-Open Publication No. 7-146515 (supra) each disclose a method for reducing the starting voltage by preventing the enclosed material in the discharge lamp from depositing on the electrodes. According to the method disclosed in Japanese Patent Application No. 6-265318 (corresponding to Japanese Laid-Open Publication No. 8-130096), the discharge lamp is turned off while the current in the lamp is gradually decreased over time, thereby causing the temperature decrease rate of the electrode to slow. Thus, the enclosed material is prevented from depositing on the electrodes, and the starting voltage is maintained low. According to the method disclosed in Japanese Laid-Open Publication No. 7-146515, additional discharge is performed for a short period when a prescribed period of time passes after the lamp is turned off, thereby scattering the particles of the enclosed material from the electrodes. Thus, the starting voltage is maintained low.
The method disclosed in Japanese Laid-Open Publication No. 51-66174 is not desirable in consideration of the effect of the radioactive material enclosed in the discharge lamp on human bodies and environment.
The methods disclosed in Japanese Patent Application No. 6-265318 (corresponding to Japanese Laid-Open Publication No. 8-130096) and Japanese Laid-Open Publication No. 7-146515 require complicated control of the power source (lighting circuit) when the discharge lamp is turned off, and thus stable control cannot be ensured.